The present invention relates to a method for manufacturing a bicolored polyhedral body of aluminum or, more particularly to a method for providing decorative coloring on the surface of a polyhedral body made of aluminum or an alloy mainly composed of aluminum in two different colors, in which the faces colored in dark and in light are decisively and beautifully demarcated by the ridgeline of the polyhedral body.
Needless to say, a diversity of articles made of aluminum or an aluminum alloy (hereinafter referred to simply as aluminum) and anodized and colored on the surface are widely used by virtue of their lightness in weight and beautifulness as well as their high anti-corrosion resistance. For example, aluminum-made watch cases have acquired a commercial success with anodization and coloring on the surface. These conventional aluminum-made watch cases are, however, colored in a single tone so that there has been an eager demand to overcome the monotonousness of the unicolored cases and to further enhance the aesthetic value of them by providing bicolored surfaces to the watch case in dark and light colors.
Such a bicolored polyhedral body, in particular, watch case, of aluminum can be obtained in principle by subjecting an anodized and colored article with a partly removed surface layer to a second anodization and coloring treatment in a different color. This conventional method is not always satisfactory because the anodized and colored surface film of the article formed in the first step is readily degraded in the second anodization and coloring treatment resulting in poor surface properties. Various attempts have been made to overcome the above mentioned problems in the prior art method of two-step anodization and coloring treatment but without noticeable success.